1. Field
Exemplary embodiments of the present invention relate to a delay control circuit for generating a latency signal and a semiconductor memory device including the same.
2. Description of the Related Art
Semiconductor devices may exchange signals and data with each other. For example, in the case of a semiconductor memory device, such as a dynamic random access memory (DRAM), if a memory controller of a system applies a read command to a memory device, the memory device outputs data stored in a memory cell region and transfers the data to the memory controller. However, the memory device does not output data immediately when it receives the read command, because a certain time is taken to prefetch and align data in the memory device and output the aligned data.
In order to facilitate mutual operations of several semiconductor devices, it is desired to define such a time measured from the input of a command into a semiconductor device to the performance by the semiconductor device in response to the command. This time is called a latency. For example, in a DRAM, a CAS latency (CL) is defined as a time taken from the input of a read command to the actual output of data. If CL=5 clocks (tCK), data is to be outputted through a data output pad (DQ) after a time corresponding to 5 clocks from the input of the read command to the DRAM. A delay control circuit serves to control a delay amount of various signals inputted to the semiconductor device so that operations relative to the signals can be performed at accurate timings based on latencies of the signals.
Meanwhile, the delay control circuit is generally used with a delay locked loop (DLL) in order to control delay amounts of signals or data outputted from the semiconductor device based on latencies of various types of input signals. The DLL includes a circuit for synchronization of clocks used in a system and semiconductor devices. For example, a semiconductor device, such as a double data rate synchronous DRAM (DDR SDRAM), uses a DLL to generate an internal clock by delaying an external clock used in an external system by a predetermined time, and transmits various signals and data using the internal clock. This is because, although the clock initially inputted to the semiconductor device is synchronized with the external clock, it is delayed while passing through several elements inside the device and therefore it is not synchronized with the external clock when outputted to the outside of the device. To transmit signals and data stably to the external system, the internal clock and the external clock are to be accurately synchronized with each other by compensating the internal clock for the time taken to load data on a bus within the semiconductor device. To this end, the DLL is used.
FIG. 1 is a configuration diagram of a conventional delay control circuit, and FIGS. 2A and 2B are operation timing diagrams of the delay control circuit illustrated in FIG. 1.
Referring to FIG. 1, the conventional delay control circuit includes a DLL 100, a signal selection unit 107, a first delay unit 109, a first replica delay unit 111, a measurement unit 113, an operation unit 115, and a latency delay unit 117. The DLL 100 includes a second delay unit 101, a second replica delay unit 103, and a phase comparison unit 105.
The overall operation of the delay control circuit includes locking a first delay amount A of the DLL 100, measuring path information N using a measurement signal CNT, and generating a latency signal CMD_LT of a driving signal CMD.
In the step of locking the first delay amount A of the DLL 100, the second delay unit 101 delays the external clock EXTCLK by the first delay amount A to generate the internal clock DLLCLK. The second replica delay unit 103 delays the internal clock DLLCLK by the replica delay amount B to generate the feedback clock FBCLK. The internal clock DLLCLK may be used in a semiconductor chip including the delay control circuit, and the external clock EXTCLK may be a system clock that is commonly used in an entire system including a semiconductor chip. The replica delay amount B refers to modeled values of time taken for the internal clock DLLCLK to reach a target circuit (not shown) within the semiconductor chip.
The feedback clock FBCLK is inputted to the phase comparison unit 105 together with the external clock EXTCLK. The phase comparison unit 105 compares phases of the two clocks EXTCLK and FBCLK and controls the first delay amount A. Through these procedures, the phase comparison unit 105 determines the value of the first delay amount A accurately by synchronizing the phases of the two clocks EXTCLK and FBCLK (at this time, the first delay amount A is called “locked”), and both of the first delay unit 109 and the second delay unit 101 have the locked value of the first delay amount A.
In the step of measuring the path information N, as illustrated in FIG. 2A, the measurement signal CNT is activated and simultaneously inputted to the first delay unit 109 and the measurement unit 113 through the signal selection unit 107. The first delay unit 109 delays the measurement signal CNT by the first delay amount A and transfers the delayed measurement signal CNT_A to the first replica delay unit 111, and the first replica delay unit 111 further delays the delayed measurement signal CNT_A by the replica delay amount B and outputs the delayed measurement signal CNT_B. The first replica delay unit 111 may be designed to be identical to the second replica delay unit 103 of the DLL 100.
The measurement unit 113 measures a difference of a delay amount between the measurement signal CNT and the delayed measurement signal CNT_B and generates the path information N. To be specific, the measurement value obtained by counting the number of toggling of the external clock EXTCLK from the point of time when the measurement signal CNT is activated to the point of time when the measurement signal CNT_B delayed through the first delay unit 109 and the first replica delay unit 111 is activated is generated as the path information N. That is, the path information N is the value representing the sum (A+B) of the first delay amount A and the second delay amount B in clock units (tCK). The path information is the value obtained by measuring the time, taken until the signal inputted to the semiconductor device reaches the target circuit, based on the clock unit (tCK).
In the step of generating the latency signal CMD_LT of the driving signal CMD, as illustrated in FIG. 2B, the driving signal CMD of the target circuit is inputted through the signal selection unit 107, and the first delay unit 109 delays the driving signal CMD by the first delay amount A and outputs the delayed driving signal CMD_A. Therefore, the delayed driving signal CMD_A is synchronized with the internal clock DLLCLK.
The operation unit 115 subtracts the path information N from the latency LT of the driving signal CMD and provides the delay information DLI (DLI=LT−N) to the latency delay unit 117. As illustrated in FIG. 2B, if the latency LT of the driving signal CMD is 5 clocks and the path information N is 2 clocks, the delay information DLI becomes 3 clocks.
The latency delay unit 117 further delays the delayed driving signal CMD_A from the first delay unit 109 by the delay information DLI (3 clocks), and generates the latency signal CMD_LT. After the time corresponding to the replica delay amount B, the latency signal CMD_LT reaches the target circuit which actually performs an operation relative to the driving signal CMD. Therefore, the target circuit can perform the operation relative to the driving signal CMD after the time corresponding to the latency LT (5 clocks) from the input of the driving signal CMD to the semiconductor device.
However, process, voltage and temperature (PVT) variations of the semiconductor device may cause an error in the first delay amount A and the replica delay amount B. Thus, the measurement value of the path information N may be smaller or larger than the practical value. In particular, it is highly likely to cause such an error when the delay control circuit is used in the semiconductor device whose frequency range is very wide, just like DDR3 or DDR4 SDRAM